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  • Articles  (90)
  • 2020-2022  (2)
  • 1995-1999  (49)
  • 1985-1989  (37)
  • 1900-1904  (2)
  • Biology  (64)
  • Chemistry and Pharmacology  (32)
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  • Articles  (90)
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  • 1
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 90 (1986), S. 902-905 
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 6949-6955 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The emission spectrum of CoF has been investigated in the 820 nm–3.5 μm spectral region using a Fourier transform spectrometer. The bands were excited in a carbon tube furnace by the reaction of cobalt metal vapor and CF4 at a temperature of about 2300 °C. The bands observed in the 3000–9000 cm−1 region have been classified into three new transitions. The bands with 0-0 R-heads at 3458 cm−1, 3759 cm−1, and 4012 cm−1 have been assigned as the 3Δ1–3Φ2, 3Δ2–3Φ3, and 3Δ3–3Φ4 subbands of the C 3Δ–X 3Φi electronic transition. To higher wave numbers, two bands with R-heads at 8396 cm−1 and 8565 cm−1 have been assigned as the 3Δ2–3Φ3 and 3Δ3–3Φ4 subbands of the D 3Δ–X 3Φi transition. In addition, the bands with R-heads at 6339 cm−1 and 6542 cm−1 have been assigned as the 0-0 3Φ4–3Δ3 and 3Φ3–3Δ2 subbands of the G 3Φ–C 3Δ transition. The G 3Φ–X 3Φ transition has been reported previously as the [10.3]3Φ–X 3Φ transition. The rotational analysis of many bands of these transitions has been obtained and the molecular constants for the two new low-lying excited states have been extracted. Six new band involving the high vibrational levels of ground state (up to v=6) have been identified in the 3Φ4–3Φ4 subband of the G 3Φ–X 3Φ transition. The rotational analysis of these bands provides improved constants for the ground state. We have noticed, as have previous workers, the strong correspondence that exists between the states of transition metal monofluorides and monohydrides. In addition, all of the low-lying states of CoF and CoH are related to the low-lying terms of the Co+ atom. We discuss these correlations between the energy levels of CoF, CoH, and Co+. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1520-5827
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 107 (1985), S. 4721-4724 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 4544-4553 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Selenium reacts with fluorine with a weak visible chemiluminescence between 3800 and 8900 A(ring). Identical spectra are obtained regardless of whether the source of selenium is the elemental vapor or hydrogen selenide. By analogy with C10, TeF, and reevaluated SF absorption data, the SeF band system is assigned to transitions between inverted 2Π states. The spectroscopic constants for the A 2Π3/2 to X 2Π3/2 system are: Te=14 680.4 cm−1, ω‘e =688.8 cm−1, ωex‘e =1.84 cm−1, ω'e =444.1 cm−1, and ωexe =2.93 cm−1. The weak 2Π1/2 to 2Π1/2 subband is described by the constants: Te =15 025.4 cm−1, ω‘e =683.0 cm−1, ωex‘e =2.11 cm−1, ω'e =436.9 cm−1, and ωexe =1.20 cm−1. Laser excitation spectra were recorded for the SeF chemiluminescence flame. The SeF fluorescence between 4200 and 5200 A(ring) is rapidly quenched with increasing total pressure. A Stern–Volmer analysis gave a zero pressure lifetime of 12.8±3.9 μs. The measured quenching rate of 2.5×106 s−1 Torr−1 is nearly gas kinetic.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 2526-2533 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A dye laser pumped Br2 B 3Π(0+u) → X 1Σ+g laser has been studied. Spectroscopic assignments have shown that lasing occurs from 10≤J'≤63 in 12≤v'≤17 using Rhodamine 590 dye. The output appeared limited to the 79–81 isotope of Br2. By utilizing stimulated emission as a monitor for laser excitation spectra, dramatic increases in the resolution were observed that exceeded the normal resolution of the dye laser. The Br2 laser operated at Br2 pressures of up to 60 Torr. A simple model to explain the characteristics of the Br2 laser is described.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 5007-5012 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Te reacts with fluorine to give a strong chemiluminescence between 3800 and 8700 A(ring). The emission spectrum contains both 2Σ+–2Πi and 2Πi–2Πi systems. TeF emission bands are of the Hund's type (a) without spin forbidden intermultiplet crossing. Spin orbit coupling of the X 2Πi state is −497 cm−1 as determined from the 2Σ+–2Πi transition. Vibrational constants are reported for all states and compare favorably with those obtained from theory.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 18 (1995), S. 0 
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: The mechanism of water-stress-induced embolism of xylem was investigated in Malosma laurina and Heteromeles arbutifolia, two chaparral shrub species of southern California. We tested the hypothesis that the primary cause of xylem dysfunction in these species during dehydration was the pulling of air through the pores in the cell walls of vessels (pores in pit membranes) as a result of high tensions on xylem water. First, we constructed vulnerability-to-embolism curves for (i) excised branches that were increasingly dehydrated in the laboratory and (ii) hydrated branches exposed to increasing levels of external air pressure. Branches of M. laurina that were dehydrated became 50% embolized at a xylem pressure potential of -1.6 MPa, which is equal in magnitude but opposite in sign to the +1.6 MPa of external air pressure that caused 50% embolism in hydrated stems. Dehydrated and pressurized branches of H. arbutifolia reached a 50% level of embolism at -6.0 and +6.4 MPa, respectively. Secondly, polystyrene spheres ranging in diameter from 20 to 149 nm were perfused through hydrated stem segments to estimate the pore size in the vessel cell walls (pit membranes) of the two species. A 50% or greater reduction in hydraulic conductivity occurred in M. laurina at perfusions of 30, 42, 64 and 82 nm spheres and in H. arbutifolia at perfusions of 20 and 30 nm spheres. Application of the capillary equation to these pore diameters predicted 50% embolism at xylem tensions of -2.2 MPa for M. laurina and -6.7 MPa for H. arbutifolia, which are within 0.7 MPa of the actual values. Our results suggest that the size of pores in pit membranes may be a factor in determining both xylem efficiency and vulnerability to embolism in some chaparral species. H. arbutifolia, with smaller pores and narrower vessels, withstands lower water potentials but has lower transport efficiency. M. laurina, with wider pores and wider vessels, has a greater transport efficiency but requires a deeper root system to help avoid catastro-phically low water potentials.
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  • 9
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Pressure probe measurements have been interpreted as showing that xylem pressures below c. –0.4 MPa do not exist and that pressure chamber measurements of lower negative pressures are invalid. We present new evidence supporting the pressure chamber technique and the existence of xylem pressures well below –0.4 MPa. We deduced xylem pressures in water-stressed stem xylem from the following experiment: (1) loss of hydraulic conductivity in hydrated stem xylem (xylem pressure = atmospheric pressure) was induced by forcing compressed air into intact xylem conduits; (2) loss of hydraulic conductivity from cavitation and embolism in dehydrating stems was measured, and (3) the xylem pressure in dehydrated stems was deduced as being equal and opposite to the air pressure causing the same loss of hydraulic conductivity in hydrated stems. Pressures determined in this way are only valid if cavitation was caused by air entering the xylem conduits (air-seeding). Deduced xylem pressure showed a one-to-one correspondence with pressure chamber measurements for 12 species (woody angiosperms and gymnosperms); data extended to c. –10 MPa. The same correspondence was obtained under field conditions in Betula occidentalis Hook., where pressure differences between air- and water-filled conduits were induced by a combination of in situ xylem water pressure and applied positive air pressure. It is difficult to explain these results if xylem pressures were above –0.4 MPa, if the pressure chamber was inaccurate, and if cavitation occurred by some mechanism other than air-seeding. A probable reason why the pressure probe does not register large negative pressures is that, just as cavitation within the probe limits its calibration to pressures above c. –0.5 MPa, cavitation limits its measurement range in situ.
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 20 (1997), S. 0 
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Water transport from the roots to leaves in chaparral shrubs of California occurs through xylem vessels and tracheids. The formation of gas bubbles in xylem can block water transport (gas embolism), leading to shoot dieback. Two environmental factors that cause gas embolism formation in xylem conduits are drought and freezing air temperatures. We compared the differential vulnerabilities of Rhus laurina and Ceanothus megacarpus, co-dominant shrub species in the coastal regions of the Santa Monica Mountains of southern California, to both water stress-induced and freezing-induced embolism of their xylem. Rhus laurina has relatively large xylem vessel diameters, a deep root system, and a large basal burl from which it vigorously resprouts after wildfire or freezing injury. In contrast, Ceanothus megacarpus has small-diameter vessels, a shallow root system, no basal burl and is a non-sprouter after shoot removal by wildfire. We found that R. laurina became 50% embolized at a water stress of –3 MPa and 100% embolized by a freeze–thaw cycle at all hydration levels. In contrast, C. megacarpus became 50% embolized at a water stress of –9 MPa and 100% embolized by freeze–thaw events only at water potentials lower than –3 MPa. Reducing thaw rates from 0·8 °C min−1 to 0·08 °C min−1 (the normal thaw rate measured in situ) had no effect on embolism formation in R. laurina but significantly reduced embolism occurrence in well-hydrated C. megacarpus (embolism reduced from 74 to 35%). These results were consistent with the theory of gas bubble formation and dissolution in xylem sap. They also agree with field observations of differential shoot dieback in these two species after a natural freeze–thaw event in the Santa Monica Mountains.
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